nsenter: clone /proc/self/exe to avoid exposing host binary to container

There are quite a few circumstances where /proc/self/exe pointing to a
pretty important container binary is a _bad_ thing, so to avoid this we
have to make a copy (preferably doing self-clean-up and not being
writeable).

We require memfd_create(2) -- though there is an O_TMPFILE fallback --
but we can always extend this to use a scratch MNT_DETACH overlayfs or
tmpfs. The main downside to this approach is no page-cache sharing for
the runc binary (which overlayfs would give us) but this is far less
complicated.

This is only done during nsenter so that it happens transparently to the
Go code, and any libcontainer users benefit from it. This also makes
ExtraFiles and --preserve-fds handling trivial (because we don't need to
worry about it).

Fixes: CVE-2019-5736
Co-developed-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Aleksa Sarai <asarai@suse.de>

libcontainer/nsenter/cloned_binary.c

@@ -0,0 +1,268 @@
+/*
+ * Copyright (C) 2019 Aleksa Sarai <cyphar@cyphar.com>
+ * Copyright (C) 2019 SUSE LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#define _GNU_SOURCE
+#include <unistd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <string.h>
+#include <limits.h>
+#include <fcntl.h>
+#include <errno.h>
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/vfs.h>
+#include <sys/mman.h>
+#include <sys/sendfile.h>
+#include <sys/syscall.h>
+
+/* Use our own wrapper for memfd_create. */
+#if !defined(SYS_memfd_create) && defined(__NR_memfd_create)
+#  define SYS_memfd_create __NR_memfd_create
+#endif
+#ifdef SYS_memfd_create
+#  define HAVE_MEMFD_CREATE
+/* memfd_create(2) flags -- copied from <linux/memfd.h>. */
+#  ifndef MFD_CLOEXEC
+#    define MFD_CLOEXEC       0x0001U
+#    define MFD_ALLOW_SEALING 0x0002U
+#  endif
+int memfd_create(const char *name, unsigned int flags)
+{
+	return syscall(SYS_memfd_create, name, flags);
+}
+#endif
+
+/* This comes directly from <linux/fcntl.h>. */
+#ifndef F_LINUX_SPECIFIC_BASE
+#  define F_LINUX_SPECIFIC_BASE 1024
+#endif
+#ifndef F_ADD_SEALS
+#  define F_ADD_SEALS (F_LINUX_SPECIFIC_BASE + 9)
+#  define F_GET_SEALS (F_LINUX_SPECIFIC_BASE + 10)
+#endif
+#ifndef F_SEAL_SEAL
+#  define F_SEAL_SEAL   0x0001	/* prevent further seals from being set */
+#  define F_SEAL_SHRINK 0x0002	/* prevent file from shrinking */
+#  define F_SEAL_GROW   0x0004	/* prevent file from growing */
+#  define F_SEAL_WRITE  0x0008	/* prevent writes */
+#endif
+
+#define RUNC_SENDFILE_MAX 0x7FFFF000 /* sendfile(2) is limited to 2GB. */
+#ifdef HAVE_MEMFD_CREATE
+#  define RUNC_MEMFD_COMMENT "runc_cloned:/proc/self/exe"
+#  define RUNC_MEMFD_SEALS \
+	(F_SEAL_SEAL | F_SEAL_SHRINK | F_SEAL_GROW | F_SEAL_WRITE)
+#endif
+
+static void *must_realloc(void *ptr, size_t size)
+{
+	void *old = ptr;
+	do {
+		ptr = realloc(old, size);
+	} while(!ptr);
+	return ptr;
+}
+
+/*
+ * Verify whether we are currently in a self-cloned program (namely, is
+ * /proc/self/exe a memfd). F_GET_SEALS will only succeed for memfds (or rather
+ * for shmem files), and we want to be sure it's actually sealed.
+ */
+static int is_self_cloned(void)
+{
+	int fd, ret, is_cloned = 0;
+
+	fd = open("/proc/self/exe", O_RDONLY|O_CLOEXEC);
+	if (fd < 0)
+		return -ENOTRECOVERABLE;
+
+#ifdef HAVE_MEMFD_CREATE
+	ret = fcntl(fd, F_GET_SEALS);
+	is_cloned = (ret == RUNC_MEMFD_SEALS);
+#else
+	struct stat statbuf = {0};
+	ret = fstat(fd, &statbuf);
+	if (ret >= 0)
+		is_cloned = (statbuf.st_nlink == 0);
+#endif
+	close(fd);
+	return is_cloned;
+}
+
+/*
+ * Basic wrapper around mmap(2) that gives you the file length so you can
+ * safely treat it as an ordinary buffer. Only gives you read access.
+ */
+static char *read_file(char *path, size_t *length)
+{
+	int fd;
+	char buf[4096], *copy = NULL;
+
+	if (!length)
+		return NULL;
+
+	fd = open(path, O_RDONLY | O_CLOEXEC);
+	if (fd < 0)
+		return NULL;
+
+	*length = 0;
+	for (;;) {
+		int n;
+
+		n = read(fd, buf, sizeof(buf));
+		if (n < 0)
+			goto error;
+		if (!n)
+			break;
+
+		copy = must_realloc(copy, (*length + n) * sizeof(*copy));
+		memcpy(copy + *length, buf, n);
+		*length += n;
+	}
+	close(fd);
+	return copy;
+
+error:
+	close(fd);
+	free(copy);
+	return NULL;
+}
+
+/*
+ * A poor-man's version of "xargs -0". Basically parses a given block of
+ * NUL-delimited data, within the given length and adds a pointer to each entry
+ * to the array of pointers.
+ */
+static int parse_xargs(char *data, int data_length, char ***output)
+{
+	int num = 0;
+	char *cur = data;
+
+	if (!data || *output != NULL)
+		return -1;
+
+	while (cur < data + data_length) {
+		num++;
+		*output = must_realloc(*output, (num + 1) * sizeof(**output));
+		(*output)[num - 1] = cur;
+		cur += strlen(cur) + 1;
+	}
+	(*output)[num] = NULL;
+	return num;
+}
+
+/*
+ * "Parse" out argv and envp from /proc/self/cmdline and /proc/self/environ.
+ * This is necessary because we are running in a context where we don't have a
+ * main() that we can just get the arguments from.
+ */
+static int fetchve(char ***argv, char ***envp)
+{
+	char *cmdline = NULL, *environ = NULL;
+	size_t cmdline_size, environ_size;
+
+	cmdline = read_file("/proc/self/cmdline", &cmdline_size);
+	if (!cmdline)
+		goto error;
+	environ = read_file("/proc/self/environ", &environ_size);
+	if (!environ)
+		goto error;
+
+	if (parse_xargs(cmdline, cmdline_size, argv) <= 0)
+		goto error;
+	if (parse_xargs(environ, environ_size, envp) <= 0)
+		goto error;
+
+	return 0;
+
+error:
+	free(environ);
+	free(cmdline);
+	return -EINVAL;
+}
+
+static int clone_binary(void)
+{
+	int binfd, memfd;
+	ssize_t sent = 0;
+
+#ifdef HAVE_MEMFD_CREATE
+	memfd = memfd_create(RUNC_MEMFD_COMMENT, MFD_CLOEXEC | MFD_ALLOW_SEALING);
+#else
+	memfd = open("/tmp", O_TMPFILE | O_EXCL | O_RDWR | O_CLOEXEC, 0711);
+#endif
+	if (memfd < 0)
+		return -ENOTRECOVERABLE;
+
+	binfd = open("/proc/self/exe", O_RDONLY | O_CLOEXEC);
+	if (binfd < 0)
+		goto error;
+
+	sent = sendfile(memfd, binfd, NULL, RUNC_SENDFILE_MAX);
+	close(binfd);
+	if (sent < 0)
+		goto error;
+
+#ifdef HAVE_MEMFD_CREATE
+	int err = fcntl(memfd, F_ADD_SEALS, RUNC_MEMFD_SEALS);
+	if (err < 0)
+		goto error;
+#else
+	/* Need to re-open "memfd" as read-only to avoid execve(2) giving -EXTBUSY. */
+	int newfd;
+	char *fdpath = NULL;
+
+	if (asprintf(&fdpath, "/proc/self/fd/%d", memfd) < 0)
+		goto error;
+	newfd = open(fdpath, O_RDONLY | O_CLOEXEC);
+	free(fdpath);
+	if (newfd < 0)
+		goto error;
+
+	close(memfd);
+	memfd = newfd;
+#endif
+	return memfd;
+
+error:
+	close(memfd);
+	return -EIO;
+}
+
+int ensure_cloned_binary(void)
+{
+	int execfd;
+	char **argv = NULL, **envp = NULL;
+
+	/* Check that we're not self-cloned, and if we are then bail. */
+	int cloned = is_self_cloned();
+	if (cloned > 0 || cloned == -ENOTRECOVERABLE)
+		return cloned;
+
+	if (fetchve(&argv, &envp) < 0)
+		return -EINVAL;
+
+	execfd = clone_binary();
+	if (execfd < 0)
+		return -EIO;
+
+	fexecve(execfd, argv, envp);
+	return -ENOEXEC;
+}

libcontainer/nsenter/nsexec.c

@@ -534,6 +534,9 @@ void join_namespaces(char *nslist)
 	free(namespaces);
 }
 
+/* Defined in cloned_binary.c. */
+extern int ensure_cloned_binary(void);
+
 void nsexec(void)
 {
 	int pipenum;
@@ -549,6 +552,14 @@ void nsexec(void)
 	if (pipenum == -1)
 		return;
 
+	/*
+	 * We need to re-exec if we are not in a cloned binary. This is necessary
+	 * to ensure that containers won't be able to access the host binary
+	 * through /proc/self/exe. See CVE-2019-5736.
+	 */
+	if (ensure_cloned_binary() < 0)
+		bail("could not ensure we are a cloned binary");
+
 	/* Parse all of the netlink configuration. */
 	nl_parse(pipenum, &config);